Such a process is disclosed, for example, in Dutch Patent Applications 7500672, 7500673, 7515009, 7607212 and 8602001, and European Patent Application 531762. The transition metal chelate, usually iron(II)-EDTA, is used to complex and thus to effectively absorb the nitrogen oxides, of which NO is very sparingly dissolved by scrubbing water that does not contain a transition metal chelate.
The known processes each involve the simultaneous removal of nitrogen oxides (mainly NO and NO.sub.2), hereinafter referred to as NOx, and sulphur dioxide, molecular nitrogen (N.sub.2) and sulphates or amide-sulphates and many other N-S compounds generally as well as N.sub.2 O being ultimately obtained. The processing of the sulphates, N.sub.2 O and nitrogen-sulphur compounds is, however, complicated and requires various subsequent treatments with associated equipment. N.sub.2 O will be emitted with the flue gas. This is an unwanted effect since N.sub.2 O is a compound known for its strong detrimental effect on the ozone layer and its strong greenhouse effect.
Another important problem is that, in the oxidizing medium, the active Fe(II) is partially converted to the much less active Fe(III) by oxygen from the flue gas or indirectly by sulphite in the scrubbing liquid. This results in high losses of the chelate. In addition, flue gas usually contains too little sulphur dioxide (sulphite) in relation to nitrogen oxides for the complete regeneration of the NO-bonded Fe(II)-EDTA complex to its active form. Such methods have therefore not yet acquired large-scale application.
In a process which is already used in practice for the removal of nitrogen oxides from flue gases, the flue gas is contacted at 300.degree. C. with ammonia (NH.sub.3) and a catalyst, in which process nitrogen is produced. This process, the so-called selective catalytic reduction (SCR) process, however, is expensive, both as a result of the high investment costs associated with the high-temperature installations and as a result of the high operational costs associated with the ammonia and the catalyst (approximately one third of the catalyst has to be replaced every year). In addition, a completely separate process is necessary for the optional removal of sulphur dioxide from the same flue gas.